In all wireless communication systems, the motion of a user will cause the handoff of the associated communication connection. How to effectively design the communication handoff technique concerns whether the communication handoff is successful and effective distribution of resources of the communication system.
In a wireless network communication system, when a mobile node (MN) performs mobile communication in the system and enters from a cell into another cell, there will be a cell handoff. The handoff can be generally divided into a hard handoff and soft handoff. In the hard handoff, the MN only connects to a base station from start to end of the handoff. The communication quality in the handoff process can't be guaranteed. On the other hand, in the soft handoff, the MN keeps contact with several base stations simultaneously and selects the strongest signal for communication. Although the communication quality during handoff is better, the required system resources are more than that in the hard handoff.
In the first conventional handoff algorithm-signal strength handoff (SSHO), base stations only detect the signal strength of MN for the handoff. Because factors like loading and stability of the communication system aren't taken into account, once a base station adds a new MN into its range, instability of fading channel will cause instability of the communication quality.
A second conventional handoff algorithm-IS95A belongs to code division multiple access handoff techniques, as shown in FIG. 1. This algorithm compares the pilot strength of a nearby base station in a neighbor list table with a fixed communication handoff add threshold to determine whether to handoff to the nearby base station. Contrarily, if the pilot strength of a service base station is lower than a fixed communication handoff drop threshold for a fixed duration, the service base station will be removed from the neighbor list table and its data communication connection. Under different communication connection qualities, however, the same communication handoff thresholds are used as the handoff trigger control mechanism. Therefore, the actual communication connection quality and the required system resources can't be effectively detected and controlled.
A third conventional handoff algorithm makes use of behavior aggregate handoff (BAHO) technique, usually the IS-95B/cdma2000 technique. An aggregate Ec/Io strength is used to set trigger thresholds. When a MN handoffs from an original service base station to a nearby base station, the MN will perform dynamic communication quality control as the linear formula shown in FIG. 2. However, because the parameter aggregate Ec/Io strength has different performance behaviors under different number of connection paths, only using the aggregate Ec/Io strength as the parameter without considering factors like downlink interference and stability caused by the loading of the communication system can't effectively maintain the connection quality. The downlink means transmission from a base station to a mobile phone.
In order to avoid an unsuccessful connection, new legs are largely added in. A new leg is added in under a bad communication connection to improve the communication quality strength. Due to the increase of downlink interference, the communication quality can't necessarily be improved. Adding a new leg may improve the connection quality but at the cost of increasing overall system interference. If the communication quality of the new leg is unstable, the improvement to the connection quality is none. In view of this, the present invention proposes a multi-slope handoff algorithm for controlling dynamic communication quality in wireless communications. Only legs with reasonable strengths are added in to produce positive contribution for the final connection quality and conform to the nonlinear relationship between the communication quality and connection channel.